Diffraction grating wire arrays in series

ABSTRACT

Arrays of substantially parallel metallic wires of micron and sub-micron dimensions are made by directionally solidifying the components of a eutectic alloy system as a thin film and subsequently removing at least one of the phases. The arrays are useful, for example, as conductors in microelectronic devices and as diffraction gratings for electromagnetic and ultrasonic waves.

This application is a division of application Ser. No. 320,995, filedNov. 13, 1981.

The present invention relates to a method for producing arrays ofsubstantially parallel metallic wires of micron and sub-microndimensions which are useful, for example, as conductors inmicroelectronic devices and as diffraction gratings for electromagneticand ultrasonic waves.

The invention herein is related to the inventions disclosed and claimedin U.S. patent applications Ser. No. 245,764, filed Mar. 20, 1981, for"Method for Producing Eutectics as Thin Films" and Ser. No. 253,985,filed Apr. 13, 1981 now U.S. Pat. No. 4,349,621 issued on Sept. 14,1982, for "Process for X-Ray Microlithography Using Thin Film EutecticMasks" which were both filed in the name of Cline, are assigned to thesame assignee as the instant application, and are herein incorporated byreference.

Microelectronics technology in general, and integrated circuittechnology in particular, has experienced a period of extremely rapidgrowth during the past two decades. The mark of increased performance,measured principally by lower power requirements and higher speeds, hasbeen decreased size of the microelectronic devices resulting fromdecreased size of the component parts or features such as the conductors(metallization) that interconnect the elements on a single substrate orchip.

The need and the search for improved devices having higher operatingfrequencies, lower power requirements, higher speeds and decreased costcontinues. No doubt, those improved devices will be marked by a stillfurther reduction in size led by reductions in the size of the componentparts or features. An example of the benefits of reduced size may befound in surface acoustic wave devices which are useful, for example, inmicrowave generators, television tuners, and radar systems. Bydecreasing the line widths or spacing between the electrodes in thearrays that form the transducers that generate and detect surface wavesfrom about 1 micron to about 0.1 micron, the operable frequency rangemay be increased from about 1000 MHz to about 10,000 MHz.

Thus, there is a need for devices having near-micron and sub-micronsized component parts and features and for methods by which thosenear-micron and sub-micron sized features can be produced reproduciblyand inexpensively.

A method for producing arrays of substantially parallel metallic wiresof near-micron and sub-micron dimensions is provided by this invention.The arrays are useful, for example, as conductors in microelectronicdevices and as diffraction gratings for electromagnetic and ultrasonicwaves. The process of the invention is relatively simple, rapid, andlower in cost compared to current pioneering processes utilizingelectron beam equipment in the direct-write-on-the-wafer mode tofabricate sub-micron sized structures. Further, in sharp contrast topresent day electron beam equipment operating in the abovedescribedmode, the speed of the process of the invention increases as thestructures become finer. Since the method of the invention produces thearrays directly, the use of intermediate materials, such as resists, andintermediate processing steps found in such conventional processingmethods as photolithography are largely eliminated.

Briefly, and in its most general aspects, the method of the inventioncomprises the steps of sequentially depositing the components of aeutectic alloy system as overlying thin planar layers on a substrate;creating a molten zone of the components bounded by the substrate andunmelted material of the components; moving the molten zone across atleast a portion of the substrate to melt the components of the eutecticalloy at the leading edge of the zone, mixing the components in thezone, and freezing the components at the trailing edge of the zone inthe form of a eutectic thin film of the eutectic composition having analigned structure of at least two phases; and selectively removing atleast one of the phases.

If the molten zone is moved at a uniform rate across the substrate, theresulting structure, briefly described, will be, for the case of a twophase lamellar eutectic, an array of substantially parallel lamellae, orwires, having the composition of the non-removed phase of the eutecticsystem separated by the width of the removed phase. If the rate at whichthe molten zone is moved across the substrate is varied, the size andspacing of the wires will vary along the direction in which the moltenzone was moved resulting in a series of arrays. Alternatively, aplurality of parallel arrays of wires having different sizes andspacings may be made by moving a first zone across the substrate at afirst rate to form a first array and subsequently moving a second zonethrough unmelted material parallel and adjacent to the previously meltedmaterial to form a second area.

The invention may be more readily and comprehensively understood fromthe more detailed description presented hereinbelow in connection withthe accompanying drawings in which:

FIG. 1 is a schematic dimensional representation of a preform consistingof a substrate having two overlying planar layers of the two componentsof a binary eutectic system situate thereon with a planar cover layerover the eutectic components;

FIG. 2 is a schematic representation in cross-section of an apparatus,including a line heater, used to form a eutectic thin film on thepreform of FIG. 1;

FIG. 3 is a schematic representation of the top surface of the preformof FIG. 1 partially converted to a eutectic thin film;

FIG. 3A is a dimensional view of the preform of FIG. 3 in partialcross-section taken along lines 3A--3A, 3A'--3A', and 3A"--3A" of FIG.3;

FIG. 4 is a schematic dimensional view in partial cross-section of alamellar thin film eutectic situate on a substrate;

FIG. 5 is a schematic dimensional representation of a portion of apreform solidified at two different sequential rates thus forming twoeutectic fine wire arrays having different interlamellar spacings andlamellae of different widths in series with a transition region betweenthe two arrays;

FIG. 6 is a schematic dimensional representation of a portion of apreform having two eutectic fine wire arrays with differentinterlamellar spacings and lamellae of different widths in a parallelrelationship with a transition region between the two arrays;

FIG. 7 is a schematic dimensional representation of the structure ofFIG. 4 following dissolution of one of the phases;

FIG. 8 is a graph of the interlamellar spacing of thin films of theAl-CuAl₂ eutectic system as a function of solidification velocity;

FIG. 9 is a scanning electron micrograph (2000X) of a eutectic fine wirearray of the invention, tilted at about 30° to the electron beam, havingwires of the aluminum-rich phase of the Al-CuAl₂ eutectic about 1.2microns wide and an interlamellar spacing of about 2.4 microns;

FIG. 10 is a scanning electron micrograph of the eutectic fine wirearray of FIG. 9 at 10,000X;

FIG. 11 is a scanning electron micrograph (10,000X) of a eutectic finewire array of the invention, tilted at about 30° to the electron beam,having wires of the lead-rich phase of the lead-cadmium eutectic about0.4 micron wide and an interlamellar spacing of about 0.6 micron.

In the practice of the invention, a eutectic alloy is first solidifiedas a thin film as described below in brief, but functional, detail andin greater detail in the above cross-referenced and incorporated Ser.No. 245,764 application and the paper "Directionally SolidifiedThin-Film Eutectic Alloys" by H. E. Cline (Journal of Applied Physics,52 (1), pp. 256-260, January 1981) which is also incorporated herein byreference. Preferably, the eutectic alloy is one which can be solidifiedto form a lamellar structure, i.e., one having alternating plate-likeregions whose compositions correspond substantially to the phases of theeutectic system. Although the present invention is not limited to binarysystems, typical suitable binary systems include, for example, thelead-tin, lead-cadmium and aluminum-copper systems.

A substrate 10, shown schematically in FIG. 1, is provided. Suitablematerials for substrate 10 include, for example, Pyrex®, alumina, mica,silica, and other materials, such as silicon, which are useful in themanufacture of semiconductor devices. Generally, substrate 10 will havemajor opposed first or top 12 and second or bottom surfaces 14 and anouter peripheral edge area 16 interconnecting the major surfaces. Edgearea 16 may define a circle or rectangle or other convenient shape. Forheat flow considerations, surfaces 12 and 14 are preferablysubstantially parallel and optically flat, smooth, and free from dirt,lint, and other artifacts.

Next, the thicknesses of the materials, i.e., components, of theeutectic system are calculated, per unit area of either major surface 12or 14 of substrate 10, as a ratio according to the following formula:

    t.sub.1 ρ.sub.1 W.sub.1 =t.sub.2 ρ.sub.2 W.sub.2 = . . . =t.sub.n ρ.sub.n W.sub.n                                       (1)

where

W₁ =weight percent of component 1 in the eutectic

W_(n) =weight percent of the n^(th) component in the eutectic

ρ₁ =density of component 1

ρ_(n) =density of the n^(th) component

t₁ =thickness of the layer of component 1

t_(n) =thickness of the layer of the n^(th) component

and converted to actual thicknesses by use of the formula:

    t.sub.film =t.sub.total =t.sub.1 +t.sub.2 + . . . +t.sub.n (2)

For a binary system, equation (1) reduces to ##EQU1## The calculationalmethod shown above is more accurate than calculations from the phasediagram based on the so-called lever rule and is, therefore, preferred.

The starting materials should be as pure as possible, preferably "4-9s"or purer, as impurities tend to disrupt the heat and mass balance of thesolidifying eutectic during the subsequent processing described belowthereby forming defects, e.g., faults. Contamination of substrate 10 andthe materials of the eutectic is to be avoided. The practice of cleanroom conditions, such as are known to those skilled in the art of themanufacture of semiconductor devices, is preferable.

The eutectic materials and substrate 10 are transferred to suitableapparatus (not shown) for the evaporation and deposition of thematerials onto substrate 10. In a vacuum, preferably less than or equalto 10⁻⁶ torr, the materials of the eutectic are evaporated and depositedsequentially in overlying planar layer-like fashion onto substrate 10.In FIG. 1 there is shown schematically layer 18 of a first component ofa binary eutectic system deposited upon major surface 12 of substrate 10and layer 20 of the second component deposited upon layer 18.Preferably, the thickness of each layer is within ±10% of thatcalculated with Equations (1) and (2), although for some eutecticsystems the acceptable tolerance may be less than ±10%.

Optionally, cover layer 22, as shown in FIG. 1, may be provided on topof the deposited eutectic components. Cover layer 22 may be provided bydepositing a refractory metal oxide subsequent to the deposition of thelayers of the eutectic material or may be another piece of the samematerial as substrate 10 laid upon the deposited components. Cover layer22 is desirable to prevent oxidation of the eutectic during subsequentprocessing. Oxidation may also be avoided by practicing the invention ina vacuum or inert atmosphere. With or without cover layer 22, acompleted preform 24 has been fabricated at this stage.

The prepared eutectic preform 24 is next placed in an apparatus, such asthat shown schematically in FIG. 2, for the next steps in thepreparation of a eutectic thin film. Typically, the apparatus of FIG. 2consists of heat source 30 capable of projecting a beam of heat 31 ontopreform 24, heat sink 32, which may be water cooled, optional thermalbuffer plate 33, and means 35 for smoothly translating heat sink 32 andpreform 24 mounted thereon beneath beam 31 at a determinable rate.Sufficient heat is applied from heat source 30 to form narrow moltenzone 40 of width W and longitudinal length L, as shown in FIG. 3.

A line heater, such as that shown in FIG. 2, has been found to be aneffective heat source 30 with suitable modifications including a linevoltage regulator to minimize power fluctuations. One such line heateris that manufactured by Research, Inc. of Minneapolis, Minn. (Catalogue#5215-10). The line heater of FIG. 2 consists primarily of lamp 36, suchas a quartz or arc lamp, situate at the focal point of a polishedaluminum elliptical reflector 37 which has cooling channels 38 therein.The commercial line heater was further modified by shutters 39 of highlypolished aluminum which effectively intensify the thermal profile ofbeam 31 thus narrowing the width of zone 40 thereby increasing thethermal gradient in zone 40.

A laser is also a suitable, although more expensive, heat source 30 thanthe line heater. The narrower molten zone 40 created by a laser makespossible both higher thermal gradients and more rapid solidificationrates in molten zone 40 and, consequently, eutectic thin films havingthinner lamellae and narrow interlamellar spacings. Replacement of theline heater with a laser as heat source 30 requires means for spreadingthe beam into a line heat source. The use of optics to slowly scan orraster the laser beam across preform 24 has been found to result inobjectionable surface perturbations. A simple non-mechanical solution isto provide a cylindrical lens in the path of the laser beam between thelaser as heat source 30 and preform 24 to convert the circular beam intoa thin line source of heat. It was found, however, that the beamintensity was not uniform along the length of the line when the lenssystem was used, i.e., there was a decrease in power at the endscompared to the center of the line of heat.

The laser beam may be scanned rapidly enough through the use of alens-mirror system, such as a rotating polygonal mirror, to create asatisfactory line source of heat. A further advantage of the use of alaser as heat source 30 is that preform 24 may be kept stationary andmolten zone 40 traversed across preform 24 by the use of additionaloptical scanning means. By keeping preform 24 stationary, it is possibleto minimize disruption of the eutectic structure by external mechanicalvibrations. The use of the rotating polygonal mirror, however, adds tothe overall cost of the system and requires that the optical system bekept in perfect alignment.

As shown in FIG. 3A, molten zone 40 will be coextensive with at least aportion of substrate 10 and cover layer 22, if cover layer 22 ispresent. If cover layer 22 is not present, molten zone 40 will rest onat least a portion of substrate 10 and will be otherwise bounded by theunmelted eutectic material layers 18 and 20 and solidified thin filmeutectic 42 on preform 24. By operating means 35, preform 24 istraversed beneath the stationary heat source thereby, in effect, movingmolten zone 40 across preform 24. As molten zone 40 traverses preform24, the component layers 18 and 20 of the eutectic are melted at leadingedge 44 of zone 40, mixed together in zone 40, and solidified attrailing edge 46 of zone 40 in the desired eutectic pattern 42. Afterthe desired amount of material has been melted and solidified, thetraversing motion of means 35 is stopped and heat source 30 is turnedoff whereupon molten zone 40 freezes in situ. In FIG. 3, molten zone 40is shown after traversing a distance Z, in the direction of arrow 48from starting location 50.

FIG. 4 is a schematic representation of the structure of solidifiedeutectic 42 viewed perpendicular to cutting plane 3A"--3A" of FIG. 3 andis typical of the two phase thin film lamellar eutectics made by themethod described above. The interlamellar spacing, λ, defined as thedistance between the center of one lamella to the center of the nearestadjacent lamella of the same type, is shown in FIG. 4 as are the widthsλ_(A) and λ_(B) of lamella 60 and 62, respectively, and the resultantfilm thickness, t_(film). The widths of the lamellae and theinterlamellar spacing is principally a function of the rate at whichzone 40 is traversed across substrate 10. The ratio of the widths of thelamellae is substantially constant and may be determined from the phasediagram in accordance with the well-known lever rule principle.Generally, the thickness of the resultant film, t_(film), is equal tothe sum of the thicknesses of the component layers. The lamellae extendbetween and terminate in the substantially parallel and generally planartop 64 and bottom 66 surfaces of the thin film. The center lines of thelamellae are substantially parallel to the thickness dimension, i.e.,the lamellae intersect top 64 and bottom 66 surfaces of the thin film atsubstantially right angles.

As noted briefly above, the interlamellar spacing λ is a function of thegrowth, i.e., solidification, rate and generally follows the empiricalrelationship

    λ.sup.2 V=constant                                  (4)

where V equals the growth rate. Generally, the growth rate is equal tothe rate at which molten zone 40 is traversed across preform 24. Byvarying the growth rate during the course of traversing molten zone 40across preform 24, a thin film eutectic having adjacent regions withdifferent lamellar spacings in series may be produced. Such a structureis shown schematically in FIG. 5 for a binary eutectic where a singlechange in growth rate has been made. In the case shown in FIG. 5, thegrowth rate has decreased in the direction of arrow 48 by a factor of 4resulting in a factor of 2 increase in interlamellar spacing. As is alsoshown in FIG. 5, the initially "fault-free" structure makes a "smooth"transition across transition region R_(T), i.e., alternate lamellae 60of one phase of the binary eutectic terminate in faults 70 and thealternate non-faulted lamellae of the same phase grow smoothly anduniformly to the new size dictated by relationship of equation (4) andthe phase diagram for the alloy involved. Viewed in the directionopposite to that of arrow 48 of FIG. 5, the lamellae 62 of the otherphase are seen to smoothly "branch" across transition region R_(T).Smooth transitions are usually observed when λ and V are integer values.The transition region will be more irregular and more extensive whennon-integer values rather than integer values are involved. The lengthof region R_(T) (in the direction parallel to arrow 48) is primarily afunction of the thermal lag of the mechanical system, whether or not thechange in V is made gradually or abruptly, and the geometry of theeutectic phases.

An alternative variably spaced structure wherein adjacent regions withdifferent interlamellar spacings are in parallel is shown schematicallyin FIG. 6. The structure of FIG. 6 would be produced by directionallysolidifying a first array or region with a first interlamellar spacing,λ₁, in the direction of arrow 48. A second array, or region, paralleland adjacent to the first region would then be solidified at a differentsolidification rate, again in the direction of arrow 48, therebyproducing a second region with a second interlamellar spacing λ₂. Inthis case, the nature and quality of the structure in the transitionregion, R_(T), will be a function of, for example, the sharpness of thethermal gradients in molten zone 40. Ideally, the transition regionwould be a sharp boundary between the two regions which were solidifiedat different rates, V₁ and V₂. Although only a single change in thesolidification rate is shown in FIGS. 5 and 6, it should be recognizedthat the solidification rate may be varied so that a plurality ofstructures of different interlamellar spacing are produced either inseries or in parallel.

It has been discovered that one of the phases of the eutectic alloy maybe selectively removed by chemical dissolution, or other means such asreactive ion etching, leaving behind an array of substantially parallelmetallic wires of micron and sub-micron dimensions useful, for example,as conductors in microelectronic devices and as diffraction gratings forelectromagnetic and ultrasonic waves. Removal may be accomplished byimmersing the solidified eutectic thin film in a chemical solution,preferably with agitation of the solution. Electrochemical dissolutionmay also be employed to selectively remove one of the phases. In theelectrochemical method, the thin film is made the anode and placedopposite an electrode of an inert material of opposite polarity. Theelectrochemical solution may be agitated or caused to flow against thethin film as in the electropolishing process.

The resultant structure, an array of substantially parallel wires isshown schematically in FIG. 7. As used herein, the term "wire" refers toa structure which is substantially, but not necessarily, rectangular orsquare in cross-section and, as shown below, may be substantially smoothand regular in appearance or may be rough depending, for example, on thedegree of attack by the dissolution medium and the magnification atwhich the wires are observed. Thus, when used in conjunction with thisinvention, the term "wire" is intended to include a metallic unit ofwhatever cross-sectional geometry whose length is long in comparison toits height (H) or width (W) (at least 100:1), having a compositionsubstantially equal to the terminal solid solution of at least one phaseof a eutectic alloy system and remaining after the dissolution of atleast one other phase of the eutectic alloy formed as a thin (less thanor equal to about 8 microns) film. If the wires are of non-rectangularcross-section, then their width (W) will be measured as the largestdimension on a plane parallel to the plane of substrate 10. Likewise,the term "array" refers to at least a pair of the wires in asubstantially mutually parallel relationship and defining a gap or emptyspace inbetween.

Since the phases of the eutectic thin film may not have a volumerelationship equal to or close to 1:1, FIG. 7 includes the terminologyof "Minimum Feature Size", λ_(mfs), sometimes referred to by thepractitioners of the semiconductor arts. The minimum feature size isdefined as the smallest feature of a pattern or a lithographic mask. Theinterlamellar spacing, λ, and the minimum feature size, λ_(mfs), will besubstantially equal only when the eutectic phases are present on a 1:1basis. In FIG. 7, where lamellae 62 and the gaps therebetween areapproximately equal, λ_(mfs) has been chosen as the distance or gapbetween substantially parallel wires 62 for purposes of illustration.When referring to the arrays of wires made by the method of thisinvention, it may be convenient to refer to the interlamellar spacing,λ, as the interwire spacing (λ_(iws)).

The fine wire array structure of FIG. 7 is useful, for example, as aconductor array for the interconnections of elements on a singlesemiconductor chip. On an optically transparent substrate the structureof constant period (λ_(iws)) of FIG. 7 is also useful as a diffractiongrating. Since the spacing between the wires can be made in thenear-micron and sub-micron range, and varied by varying the rate atwhich the eutectic is solidified, the gratings of the invention areparticularly useful in the ultraviolet region of the electromagneticspectrum. On a substrate which is transparent to X-radiation, thegratings of FIG. 7 of the invention are particularly suited for use asmasks in X-ray lithography as described in more detail in the abovecross-referenced and incorporated Ser. No. 253,985 application and thepaper, "Submicron-Resolution Eutectic Thin Film Mask" by H. E. Cline inApplied Physics Letters (37 (12), Dec. 15, 1980, pp. 1098-1101) which isherein incorporated by reference.

Similarly, by the method of the invention, the structures of FIGS. 5 and6 may be made as arrays of wires having the appearance of the phaseremaining, e.g., lamellae 60, after the other phase, lamellae 62, hasbeen selectively removed. The multiple array structures of FIGS. 5 and 6generically describe diffraction gratings for electromagnetic oracoustic waves. More particularly, lamellae 60 (or 62, depending uponwhich phase is selectively removed) of FIG. 5 are in the configurationof a spectrum analyzer and lamellae 60 (or 62, depending upon whichphase is selectively removed) of FIG. 6 are in the configuration of afilter. The unique nature of the transition region, R_(T), of thestructure of FIG. 5 renders it difficult, if not impossible, to produceusing standard techniques and so-called ruling engines.

The following Examples are provided by way of illustration, and not byway of limitation, to further instruct those skilled in the art of themanner of practice of the invention.

EXAMPLE I

Several preforms of the Al-CuAl₂ eutectic alloy system (33.2 wt.% Cu)were made by depositing layers of Cu and Al on a Pyrex® substrate withan electron beam evaporator in a vacuum of 10⁻⁶ Torr at a rate of 20Angstroms/sec. During evaporation, the thicknesses of the layers weremonitored to give a 2580 Å thick copper layer and a 17420 Å thickaluminum layer. These thicknesses were calculated by equation (3) togive a 2 micron thick thin film of the eutectic composition.

A first set of three preforms was directionally solidified at 0.0016 and0.0042 cm/sec using the line heater means described above equipped witha quartz iodine lamp having a beam focused to provide a 3 mm wide lineof heat and a substantially equally wide molten zone.

With the line heater means, the thermal gradient in the molten zone wasinsufficient to yield aligned lamellar structures at solidificationrates greater than about 0.0042 cm/sec. Therefore, the remainingpreforms were solidified using laser beam means wherein the heat from a90 watt (Continuous Wave) Nd:YAG laser was focused with a 50 mm focallength cylindrical lens to a 0.02 cm wide line forming a substantiallyequally wide molten zone.

The width of the aluminum-rich phase (α) was equal to the width of theCuAl₂ phase (θ); consequently, the Al-rich wires were one-half as wideas the interlamellar spacing. A graph of lamellar spacing as a functionof the growth or solidification velocity is shown in FIG. 8, and isdescribed by the equation

    λ=B(V).sup.-1/2                                     (5)

where the constant B=8.4×10⁻⁶ cm^(3/2) sec^(-1/2). The scatter in theinterlamellar spacing is attributed to local fluctuations in thesolidification velocity or in the constancy of the heat flow.

The CuAl₂ phase was selectively removed by electropolishing the filmssolidified at 0.0016 cm/sec in a Disa Electropol Type 53 apparatus in aflowing solution of 62 ml perchloric acid, 137 ml water, 700 ml ethanol,and 100 ml butycellosolve. The electropolishing process was conducted at35 volts direct current (DC) for 4 seconds at room temperature with thethin film as the anode and a stainless steel cathode of approximatelythe same surface area as the film situated opposite the thin film andseparated by about 10 mm from the film.

The resulting structure of the array of aluminum-rich (α-phase) wires ofthe film solidified at 0.0016 cm/sec is shown in the FIG. 9 scanningelectron micrograph at 200X wherein the array is tilted at about 30° tothe beam to enhance contrast. Measurements on untilted specimens showedthat the wires were substantially rectangular in cross-section and wereabout 1.2 microns wide by about 2 microns high and that theinterlamellar spacing was about 2.4 microns. As may be observed in theFIG. 10 scanning electron microscope at 10,000X of the same general areaas FIG. 9, some etching of the Al-rich phase has occurred which resultedin surface roughening of the Al-rich wires.

Transmission electron microscopy of the as-solidified eutectic thin filmdemonstrated that the interphase boundary between the Al-rich phase andthe CuAl₂ phase was planar and oriented normal to the plane of the film.Selective area electron diffraction was used to identify the phases andshowed that the fine wires were single crystals. Single crystals areadvantageous for use as metallization at submicron dimensions tominimize electromigration and improve thermal stability.

Measurements of the preforms solidified at 0.14 cm/sec indicated thatthe lamellae of both phases were about 0.16 micron wide and theinterlamellar spacing was about 0.33 micron. Ultimately, the minimumfeature size of the arrays of wires of the aluminum-copper eutecticsystem that may be obtained by the method of this invention is about 100Å which is that reported in the literature for a splat cooled thin foilof the Al-CuAl₂ eutectic. At solidification rates more rapid than thosefound in splat cooling, the eutectic alloy liquid was reported totransform to a single non-equilibrium solid solution.

EXAMPLE II

Using the equipment and techniques described above in Example I,additional preforms of the Al-CuAl₂ eutectic system were prepared andsolidified at 0.0042 cm/sec. Whereas the thicknesses of the layers ofaluminum and copper of Example I were maintained to yield a resultantthin film of nearly exact eutectic composition (33.2 wt.% Cu), thethicknesses of the layers of aluminum and copper of this Example werevaried to yield thin films having 34.9, 33.9, 32.0, and 31.0 weightpercent copper. Those preforms having 34.9 and 31.0 weight percentcopper solidified to form dendritic structures whereas those preformshaving 33.9 and 32.0 weight percent copper solidified to formsubstantially uniform lamellar structures. Thus, the thicknesses of thelayers of the components of the Al-CuAl₂ eutectic system may vary suchthat the composition of the resultant thin film is within about ±2.0percent of the eutectic composition and still be solidified to formlamellar thin film eutectic structures.

EXAMPLE III

Using the same general procedure and equipment of Example I, a 5320 Åthick layer of cadmium and a 15,680 Å thick layer of lead weresequentially deposited on a Pyrex® substrate in a vacuum of 10⁻⁶ Torrand directionally solidified at 0.0042 cm/sec.

The resultant lead-cadmium eutectic (17.4 wt.% Cd) thin film waselectropolished using the pump from the Disa apparatus to pump asolution of 1% perchloric acid in methanol through a hypodermic syringe.The needle of the syringe as the cathode was traversed back and forthacross a selected area of the anodic thin film at a distance of about 4cm at a direct current potential of about 150 volts between the two. Thestructure was observed periodically through a microscope and theelectropolishing was continued until the lamellae of the cadmium-richphase were removed from between the lamellae of the lead-rich phase inthe selected area. The resultant array having lead-rich wires about 0.42micron wide and an interlamellar spacing of about 0.57 micron is shownin the 10,000X scanning electron micrograph of FIG. 11 wherein the arrayis tilted at about 30° to the beam to enhance contrast. Alternatively,it has been found that dissolution of the cadmium-rich wires can beaccomplished by simple immersion in a 1% solution of Nital (1 ml HNO₃ in99 ml ethyl alcohol).

While the invention has been particularly shown and described withreference to several preferred embodiments thereof, it will beunderstood by those skilled in the art that various changes in form anddetail may be made therein without departing from the true spirit andscope of the invention as defined by the appended claims.

What is claimed is:
 1. A diffraction grating situate on a substratecomprising a plurality of eutectic fine wire arrays in series, saidgrating comprising:(1) a first region of substantially mutually parallelwires situated on said substrate, said wires of said first region havinga composition substantially corresponding to at least one phase of aeutectic alloy system, a first interwire spacing and a first widthmeasured along a plane parallel to the plane of said substrate; (2) atleast a second region of substantially mutually parallel wires situatedon said substrate, said wires of said second region having the samecomposition as the wires of said first region, a second interwirespacing and a second width measured along a plane parallel to the planeof said substrate; and (3) a transition region between said first andsecond regions, at least some of said wires of said second region beingcontiguous with at least some of said wires of said first region acrosssaid transition region.
 2. The grating of claim 1 wherein saidcomposition of said wires corresponds substantially to the aluminum-richphase of the Al-CuAl₂ eutectic system.
 3. The grating of claim 1 whereinthe material of said substrate is one selected from the group consistingof glass, mica, silicon, and alumina.
 4. The grating of claim 1 whereinsaid composition of said wires corresponds substantially to thelead-rich phase of the lead-cadmium eutectic system.